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Following is an article that I wrote last year. I have also included an
Excel spreadsheet where you can plug in your own engine size and scoop
diameter. Lorn H. Olsen
Lorn,
The calculations you presented appear to recommend the inlet to be sized for
a 1:1 velocity ratio - in other words the velocity in the inlet is equal to
the free airstream velocity. I see other recommendations that are for
velocity ratios between 0.4 and 0.7, producing much of the pressure recovery
ahead of the inlet. The advantage of that, as I see it, is that pressure
recovery ahead of the inlet is quite efficient and requires less recovery
inside the duct, reducing the required length. Also, at lower airspeeds the
inlet is more efficient as there is less chance for flow separation inside
the duct. What do you think?
Gary Casey
Gary,
My calculations only have to do with the straight forward mathematics used to calculate the diameter of the scoop. I make an assumption that there is no air compression and no air friction.
In order to calculate the most optimum size of the scoop when both air compression and friction are considered, the equations become much more complex. I think, so complex in fact, that wind tunnels should be used. I have neither a wind tunnel nor the thermodynamic engineering background to do these more advanced studies (calculations).
My spreadsheet does not recommend a specific scoop diameter (velocity ratio). The spreadsheet simply shows, based upon the scoop diameter, the extra percentage of air that you, the builder, have chosen to incorporate into your design.
--
Lorn H. 'Feathers' Olsen, MAA, DynaComm, Corp.
248-345-0500, mailto:lorn@dynacomm.ws
LNC2, O-320-D1F, 1,000 hrs, N31161, Y47, SE Michigan
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